The TCR zeta chain, either in the native or a mutated form, has been transfected into zeta chain-negative variants of the murine T-cell hybridoma 2B4.11. Six different zeta species have been studied in detail. All forms of zeta restored cell surface TCR levels to normal levels. The full length zeta transfectant (FL) was able to respond to antigen or anti-CD3 antibodies by producing IL-2 and being growth inhibited. A transfectant in which zeta was truncated at residue 150 (clipping off the last 14 amino acids of the chain) responded well to anti-CD3, but poorly to antigen. Another transfectant in which zeta was truncated at residue 108 responded only to anti-CD3; no IL-2 or growth inhibition was found when antigen was used as the stimulus. Two point mutations resulted in the same phenotype. GV135 is a cell line in which a Gly->Val substitution was made at residue 135; YT153 is a transfectant in which a Tyr->Thr substitution was made at residue 153. These substitution would be predicted to alter the consensus nucleotide-binding site and the ultimate phosho-tyrosine, respectively. In contrast, a substitution of a Lys for an Arg at residue 150 resulted in a transfectant that was indistinguishable from the wild type zeta chain in its biological responses to activation. These data indicate that the zeta chain plays a crucial role in transmembrane transduction of activating signals. Furthermore, they indicate that occupancy of the TCR with ligand is qualitatively dissimilar from cross-linking with multivalent reagents. The difference between antibody and antigen could not simply be due to the different TCR chains involved, since antibodies directed to TCR alpha or TCR alpha-beta were identical to anti-CD3 antibodies in this regard. Finally, in contrast to zeta, transfection of delta chains that had the entire intracytoplasmic portion truncated yielded cells that responded as well as cells transfected with the full length delta chain. These data emphasize the modular nature of the TCR complex, and strengthen the model that different chains or sets of chains subserve specific and discrete functions.